Strain sensors are employed for measuring the shear strain on a structure. Conventional strain sensors use a flexible backing that supports a metallic foil pattern and are mounted directly to a structure to be tested. When the structure is deformed due to shear strain, the metallic foil is deformed, which alters the electrical resistance of the foil. By measuring the electrical resistance across the terminals of the foil, the strain of the structure may be measured. However, the metallic foil may become permanently deformed or detached from the backing over time.
Vibrating wire gauges have also been be used to detect strain. A vibrating wire sensor measures force using a wire that vibrates at a high frequency. The applied external force changes the tension on the wire and this changes the frequency. The frequency is measured and indicates the amount of force on the sensor. The load sensor can have an integrated electronic system to both activate the vibrating wire as well as to read the frequency. The strain is calculated by measuring the resonant frequency of the wire wherein an increase in tension increases the resonant frequency. However, these gauges must be connected directly to the structure to be measured or embedded in the structure to be measured wherein they can be damaged during installation and can be difficult to utilize after the pile and/or structural object is in operation.
U.S. Pat. No. 8,161,823 to Berris overcomes many of the problems in the prior art by utilizing a capacitively-coupled strain sensor, which includes a first board and a second board, both with conductive pads. An insulating layer is included between the boards to create a capacitive network between the conductive pads on the boards. In Berris, provided is a capacitive network created by the boards and the insulating layer that creates a capacitive full bridge. In certain embodiments, the first board includes an oscillatory signal driver that produces an oscillatory signal, which is fed to conductive coupling pads on the first board. The oscillatory signal passes to the second board via the coupling capacitors of the capacitive network and returns to the first board via the signal capacitors of the capacitive network. The resulting signal is indicative of the amount of strain on the structure and/or the displacement of one board relative to the other board. The sensor can thus be used to perform a strain sensing function and/or a displacement calculation without a conductor linking the two boards. However, while Berris has been found to be an effective strain gauge, it still requires direct contact with the object to be measured. In this respect, while prior art sensors are effective, they require the sensors to be either embedded into the structural object or attached to the structural object. Sensors that are embedded into the structural object are expensive since the sensor can only be used once since it remains in the structural object. While sensors that are attached to the structural object can be re-used, it is time consuming to properly attach, remove, unattach, and reattach each sensor from the structural object. Frequently, adhesives cannot be used since adhesives take too long to set, are impractical to apply in adverse weather conditions such as in rain or extreme cold conditions, and they are not strong enough for the shear stresses associated with strain measurements in a dynamic environment. More importantly, using adhesives to attach the strain sensors has been found to be less accurate. Therefore, the sensors must be bolted to the structural object, which is time consuming and potentially damaging to the structure. Yet further, having to bolt a sensor to the structural object means that only one location of the structure is tested unless multiple sensors are mounted to the structural object or the sensors are repeatedly removed and moved to different locations, which is especially difficult and time consuming for the testing of driven piles. Moreover, the locations in which the sensors must be attached can be difficult to access. Thus, there is a continuing need for a strain sensor and/or strain sensor system that eliminates the current requirement to make a mechanical connection between the sensor and the object to be tested.